In hydrometallurgical processes, there are often produced solutions which must be worked up to recover the desired end products such as metals or metal salts. The steps whereby the metal or metal salts are recovered from such aqueous solutions are difficult to achieve in the presence of one or more of the impurity elements arsenic, antimony and bismuth.
For the removal of these impurity elements from such solutions, it is known to evaporate a part of the solution to precipitate the metal salt whereby a portion of the impurities remain in the mother liquor or residual solution, while a portion of the impurities are found in the precipitated metal salt. The impurities may be removed from the mother liquor by an electrolytic separation while the impurities can be removed from the precipitate by recrystallization (see Engelhardt "Die technische Elektrolyse wassriger Losungen", Teil A, Seite 100 bis 113, Akademische Verlagsgesellschaft, Leipzig, 1932; Tafel, Lehrbuch der Metallhuttenkunde, Bd. 1, Seite 560, Seite 568 ff., S. Hirzel Verlag, Leipzig, 1951).
Another known method involves the precipitation of these impurity elements as their sulfides (see H. Saarinen: Nickel Symposium 1970, Seite 13, Gesellschaft deutscher Metallhutten-und Bergleute, Clausthal). This process is limited to cases in which the desired metal will not precipitate during sulfide precipitation of the impurity elements.
In still another method, the pH value of the solution is sharply altered so that the impurity elements are precipitated as simple compounds (A. Lange, Erzmetall 18 (1965) Heft 12, Seite 613 "Hydro- und elektrochemische Zink- und Cadmiumgewinnung").
With large pH changes it is necessary to use large quantities of reagents, such as acids or bases, which increase the expense of the process and cause an enrichment in the solution with neutral salts. Such enrichment is often not desirable.
Still another conventional process has the function of preventing a supersaturation of an electrolyte with one or more of the impurities arsenic, antimony and bismuth. In these processes the electrolyte is brought into contact with a high-surface chemisorption agent (German published application (Auslegeschrift) 20 04 410 (corresponding to U.S. Pat. No. 3,696,012) and German published application-Auslegeschrift- 22 18 934). The chemisorption agent is usually a low-solubility metal oxide hydrate, especially stannic acid. Especially good results with this process are obtained with systems in which the chemisorption agent is deposited on a substrate as described in German printed application-Offenlegungsschrift 21 25 781 and German published application-Auslegeschrift- 22 18 900.
While the latter processes have considerable advantages over the earlier art, they also involve certain disadvantages in that the regeneration of the chemisorption must be carried out with an acid of a concentration which does not cause solubilization of the metal oxide hydrate. With permissible acid concentrations, the recovered concentration of arsenic, antimony and/or bismuth in the regenerating solution is relatively low so that a proportionately large volume of acid must be used and worked up to remove the impurities elements therefrom. A further disadvantage is that the metal oxide hydrate can only be applied to the substrate in a coherent manner with difficulty and is not always easy to apply in a uniform active form.